Spatial filters are used for isolating the source of a signal from a plurality of signal sources, each transmitting a signal having approximately the same frequency. Spatial filters are commonly phased array systems which include an array of antenna elements for receiving the plurality of signals from the far-field. Typically, these antenna elements are arranged so that they are equidistant and lie along a straight line. The actual distance between the elements often depends on the wavelength of the far-field signals. Usually, that separation is approximately one-half wavelength. This arrangement of elements forms an aperture which is used to sample the incidence of electromagnetic or acoustical energy in the environment. This energy induces a sinusoidal signal voltage at each of the elements which, at some observational instant, may be represented by a complex value. The amplitude of this value is proportional to the amplitude of the induced sinusoidal signal voltage, while the phase of the complex value corresponds to the phase angle of the induced sinusoidal signal. The representation of sinusoidal signals by complex values is commonly known as a "complex envelope" representation which is made relative to a convenient frequency and phase reference.
There are a multiplicity of induced sinusoidal signals at each element of the array, one signal for each signal source in the far-field. The phase progression of the induced sinusoidal signals, from element-to-element, for a given signal source depends upon the spatial angular location of that source. As the angular position of a signal source moves from a position perpendicular to the axis of a linear array of elements (broad side) toward a spatial position along the axis of the line array (end fire), the element-to-element phase progression increases. Thus, the element-to-element phase progression may be used as a discriminant in a spatial filter to isolate signals coming from different far field sources.
Signal sources located at a particular angle are detected by properly weighting the array of data values, or voltage induced at each element, and then summing the results. Generally, weight sets are applied to compensate for the phase shift of the induced voltage at each element. Amplitude weighting is also used at times to modify the response of the spatial filter. For example, a weight set whose amplitudes tend to decrease from the center of the array of data values to its ends causes the spatial filter side lobe levels to be reduced, but at the expense of a wider main beam.
Phase shift is a factor which corresponds to a phase progression of an incident plane wave from a given far-field source as it strikes the elements. If the phase of the voltage induced at each of the elements is properly phase compensated by a weight set, the sum of the array of signal data values produces a dramatic response which corresponds to the signal source at the angle corresponding to the phase-compensation procedure. By changing the weight set, different signal sources at different angles can be located using the same array of element data values.
One problem associated with this classical method of spatial filtering (conventional beam forming) arises from the limitations imposed by an array aperture having a limited number of elements, and consequently a limited aperture size. Such limitations control the narrowness of the main beam and the depth of suppression in the side lobe regions of the spatial filter. Using this limited aperture, it becomes difficult to isolate the signal from far-field sources that are closely spaced in angular position. It also becomes difficult to isolate the signals from widely spaced sources when there is a large signal level difference between signals. Further, a very strong signal in the side lobe region may produce significant interference at the output of the spatial filter because of the limited side lobe suppression available from the spatial filter.
A second problem associated with classical spatial filtering or conventional beam forming involves the fact that the spatial filter, designed to isolate the signal from one particular far-field source, must contend with induced signal voltages from the array elements caused by other far-field sources. Since the time of the invention of wave filters by the Bell Telephone Laboratories at the turn of the century, this second problem has generally been accepted as a given. Our invention demonstrates that this second problem may be avoided with dramatically improved results.